Commonly-owned U.S. Pat. No. 8,727,763 to Shanley, I V (the “'763 Patent”), details apparatus and methods for, among other things, selectively thermoforming plastics. The apparatus may include a mold comprising “a generally planar upper surface from which one or more plugs protrude.” in use, the “plugs function as three-dimensional objects about which sheets of polymer material are formed.” See '763 Patent, col. 3, ll. 33-37.
Consistent with disclosure of the '763 Patent, a to-be-formed sheet may be placed between the upper surface of the mold and an energy sink. One such sink may comprise a generally planar plate with one or more cut-outs existing through its depth. Preferably, each cut-out “is approximately the size and shape of an associated plug” protruding from the upper surface of the mold. See id., col. 3, ll. 57-67; col. 4, ll. 29-32 and 49-51. To form portions of the sheet.                [a] heater is repositioned closely above [the] sink and activated for a selected period of time . . . . Heat or other energy from [the] heater transfers to [the] sink and, where cut-outs in [the] sink exist, to . . . areas of the sheet therewith aligned, where it is absorbed by the polymeric material [of the sheet].                    As the exposed areas of the sheet absorb sufficient heat to reach their forming temperatures, [the] mold is evacuated so as to stretch (form) the material around [the] plugs. [The h]eater then may be repositioned away from [the] mold, the formed sheet of material may be allowed to cool, and [the] sink may be removed so as to expose the sheet of material.See id., col. 4, ll. 53-65 (numerals omitted).                        
Commonly-owned U.S. Patent Application Publication No. 2013/0200551 to Shanley, I V, et al. (the “'551 Application”) describes additional apparatus and methods for selective thermoforming. Included in apparatus of the '551 Application is a spacing device initially separating a sheet from the mold surface. In some versions the spacing device may be “in the form of a grid or support rail” designed to “support [the] sheet of polymeric material a substantially uniform distance from the mold surface while enabling a profile plate to block heat or other energy from an energy source to effect the selective thermoforming.” See '551 Application, p. 1, ¶ 0006. As described in the '551 Application:                The grid further may be supported by springs or other biasing mechanisms and accepted by a receiver groove or recesses in the mold. When compressed by vacuum pressure and the force of the clamps, the grid may retract flush into the mold.See id. The entire contents of both the '763 Patent and the '551 Application are incorporated herein by this reference.        
The apparatus, systems, and methods of the '763 Patent and '551 Application work well for their intended purposes. At times, however, to-be-formed polymeric sheets may lack uniform thicknesses. Also, the profile plates or other energy sinks may be subject to (generally slight) warpage or bowing. Either or both issues may cause distortion of the formed products, even in their unprinted areas.
For example, unduly thick sheets may prevent satisfactory formation of the polymeric material around the plugs of the mold. By contrast, overly thin sheets may result in excessive thinning and consequent distortion of formed areas. Accordingly, sometimes need exists for enhancing uniformity of pressure placed on the sinks and sheets of polymeric material so as to help account for these issues.